/Engine/Plugins/FX/Niagara/Source/NiagaraEditor/Public/NiagaraGraph.h
Walk through the graph for an ParameterMapGet nodes and see if any of them specify a default for VariableName.
[UEdGraphPin](API\Runtime\Engine\EdGraph\UEdGraphPin) * FindParameterMapDefaultValuePin ( const [FName](API\Runtime\Core\UObject\FName) VariableName, [ENiagaraScriptUsage](API\Plugins\Niagara\ENiagaraScriptUsage) InUsage, [ENiagaraScriptUsage](API\Plugins\Niagara\ENiagaraScriptUsage) InParentUsage ) const